I resumed heart-rate training this fall, after several years of recovering from a back injury. I wrote this summary of heart-rate information in part to remind myself of the major concepts:

Your resting heart rate is as slow as your heart naturally beats. Measure it right when you wake up, still lying in bed. My resting heart rate is about 50 beats per minute. This number increases with age and can decrease if you get more fit. If you are working out it’s good to measure your resting heart rate every morning, because if it jumps by 10% or more it’s a sign that you may have overdone it in yesterday’s workout. For example, I overdid it last Friday and my resting heart rate was 61. I took the day off.

Your heart beats faster as you get more active, of course, to serve your more active muscles. My heart rate gets to 60 or so just sitting, and 70 or so walking around.

Your maximum heart rate is the fastest your heart naturally beats. This number comes down as you age. Measure it by working out really really hard with a heart rate monitor on and seeing how high you can get it. Alternatively, if you are not in good enough shape to really push it yet, you can calculate a theoretical maximum heart rate by subtracting your age from 220 if you’re male or 226 if you’re female. There is some controversy about the accuracy of this equation, but I looked into it and the controversy looks like hair-splitting to me. The main thing to keep in mind is that if you calculate a theoretical maximum heart rate, it is not your actual maximum heart rate. My theoretical maximum heart rate at 41 is 179 beats per minute, which I hit during the aforementioned Friday workout, but the highest I’d seen it go before that this year was 165.

You can see that your range of heart rates you can narrows as you age, from the bottom and the top. I wonder if measuring your range of heart rates would be a good way to measure your biological versus chronological age, the way focal length is.

Heart rate recovery: One way to measure how in shape you are is to check how fast your heart rate descends once you’ve got it up by exercising. (Some do use this measurement as a real-versus chronological age indicator.) Just get it up pretty high, stop exercising, and see what it gets down to in one minute. My heart rate comes down about 40 beats in a minute, which is considered good.

Training zones: Exercises that cause your heart to beat at different rates have different physiological effects on your body:

Between 60 and 70% of your maximum heart rate is a mild aerobic “zone,” which increases your number of mitochondria, your capillary network density, and your efficient use of energy. According to the system I use (laid out in the book SERIOUS Training for Endurance Athletes), about 80% of your training hours are done in this zone. For me, it is between 108 and 125 beats per minute, which I experience as real exercise (I can’t get to it by walking, even very quickly) but pretty easy. I can follow the narrative of a podcast with no problem, for example, and in university I read many of my journal articles on an elliptical machine in this zone. (While I’ll give you my experience of these zones, keep in mind that you cannot use your subjective experience to judge your heart rate. Even the pros have to measure it.)

The rest of my training hours are divided in different ways between three other zones, depending on where I am in my year. Early on is mostly in the first zone and I gradually add in more of the other three.

The first of those is between 71 and 75% of maximum heart rate, and is a more intense aerobic zone than 60-70%, and has similar physiological effects, increasing endurance. I experience it as a sustainable pace, but not easy. I can no longer read and have more trouble following any narrative.

The second is between 81 and 90% of maximum heart rate, and is quite intense, used for short periods, like in sprints or interval training. That’s 144-161 beats per minute for me. I have trouble keeping it up for more than a few minutes. One thing that this kind of exercise does is essentially teach your fast-twitch muscle fibers to burn oxygen better, which lets them last longer. Exercise in this zone is called “anaerobic threshold training,” because it is the zone just before you hit the point that your heart and lungs really can’t keep up with your muscles. Staying in this zone can increase the heart rate at which you “go anaerobic,” or largely stop burning oxygen.

The third is the “anaerobic zone,” between 91 and 100%, which feels like all-out effort. Your heart and lungs can’t keep up the oxygen supply and can’t take the lactic acid away from the muscles quick enough. Your arms and legs get rubbery feeling pretty quickly. Training here is exhausting but can increase your speed and coordination.

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